Fuel cells are increasingly being used as a power source in a wide variety of different applications. Fuel cells have been proposed for use in automotive vehicles as a replacement for internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to an anode of the fuel cell and oxygen is supplied as an oxidant to a cathode. The fuel cell converts the chemical energy in the hydrogen to electrical power to charge capacitors or batteries or to directly power a device such as an electric motor.
For vehicle applications, it is desirable to use a liquid fuel, such as methanol (MeOH), gasoline, diesel, and the like, as the source of the hydrogen. Such fuels are readily available and may be conveniently stored onboard the vehicle. Fuel cells, however, cannot use the liquid fuel most efficiently without processing, via some method of dissociation, to release the hydrogen from the fuel. The dissociation reaction typically takes place within a primary reactor, which is part of the fuel cell's fuel processor. The primary function of the fuel processor is to provide a controlled hydrogen-rich stream to the fuel cell. The fuel processor produces a reformate stream that is composed primarily of hydrogen, carbon dioxide, nitrogen, water, methane and trace amounts of carbon monoxide.
The fuel processor's primary reactor has a catalyst mass for producing a reformate gas comprising primarily hydrogen and carbon dioxide. Known methods for producing the reformate include partial oxidation, steam reforming, and a combination of the two processes referred to as autothermal reforming. Partial oxidation is an exothermal reaction that produces hydrogen, carbon monoxide and heat as byproducts of reacting liquid fuel with oxygen. Steam reforming is a endothermic reaction that produces hydrogen and carbon monoxide as byproducts of reacting liquid fuel with water, which is typically in the form of steam. Autothermal reforming combines partial oxidation with steam reforming through multiple reactions.
Two important considerations when using fuel cells to power automotive vehicles is (1) the time required to start the fuel cell's fuel processor and (2) how efficient the fuel processor is at producing hydrogen. Automotive drivers are accustomed to simply turning a key and immediately being on their way. There is generally no need to wait for a conventional internal combustion engine, which power the vast majority of modern automotive vehicles, to reach a certain operating temperature before proceeding. Unfortunately, the same is not always true for vehicles powered by fuel cells. The time required to start the fuel processor may be quite significant depending on the particular process that is used to produce the hydrogen. To further complicate matters, the hydrogen producing process that has the best start-up characteristics is generally the least efficient at producing hydrogen.
Typically, the fuel processor subsystem in the fuel cell system has a “warm-up period” during which the vehicle can only be operated at a significantly reduced power or not at all. This is due to the fact that certain processes, such as steam reforming, will not begin producing hydrogen until after the reactor's catalyst bed has reached a certain minimum temperature. Steam reforming is an endothermic reaction that requires heat input for the reaction to occur. Because the reaction is endothermic, the heat must be supplied from an external source. Depending on how efficient the heat transfer is, this process can significantly affect the time required to reach the minimum temperature required for steam reforming to occur. As a result, steam reforming exhibits slower transient and startup response characteristics than either partial oxidation or autothermal reforming which are both exothermic. Steam reforming, however, can be more efficient than either partial oxidation or autothermal reforming in terms of producing hydrogen.
In contrast to steam reforming, partial oxidation has significantly better transient and startup response characteristics. This is due in part to the fact that partial oxidation is an exothermic reaction that generates its own heat. Partial oxidation, however, is considerably less efficient at producing hydrogen than steam reforming.
The performance characteristics of autothermal reforming fall somewhere between partial oxidation and steam reforming. Autothermal reforming has better transient and startup characteristics than steam reforming, but not as good as partial oxidation, and is more efficient at producing hydrogen than partial oxidation, but not as efficient as steam reforming can be.
Of the three processes—partial oxidation, steam reforming, and autothermal reforming—partial oxidation provides the best start-up transients and steam reforming is the most efficient at producing hydrogen. To take advantage of the benefits of both processes, it is desirable to develop a method for starting a steam reforming reactor that exhibits the good transient and startup characteristics of partial oxidation while also enabling the high hydrogen production efficiency of steam reformation during normal operation.